The present invention relates broadly to electromagnetic interference (EMI) shielding and more specifically, but not exclusively, relates to EMI shielding for portable electronic devices such as cellular or radio telephones.
The normal operation of electronic equipment such as computers, communications equipment, portable electronic devices such as cellular or radio telephones, and the like is attended by the generation of electromagnetic signals within the electronic circuitry of the equipment. Such electromagnetic signals often develop as a field or as a transient within the radio frequency band of the electromagnetic spectrum, i.e., from between about 10 KHz and 10 GHz, and is termed electromagnetic interference or EMI as being known to interfere with the proper operation of the electronic circuitry of other proximate electronic devices. Cellular or radio telephones in particular are required by law to adhere to Electromagnetic Compatibility (EMC) limits as laid down in Type Approval Specifications for Mobile Phones. xe2x80x9cEMCxe2x80x9d is defined as the ability of a device to function properly in its intended electromagnetic environment and not to be a source of electromagnetic pollution to that environment.
To reduce or attenuate the effects of EMI, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate the device or other target devices from other source devices. Such shielding is provided as a barrier which is inserted between the source and the other devices, and is typically configured as an electrically conductive and grounded housing which encloses the device. As the device generally must remain accessible for servicing or the like, most housings are provided with openable or removable accesses such as doors, hatches, panels, or covers. Typically, there are gaps between the accesses and the corresponding mating surfaces which reduce the efficiency of the shielding by presenting openings through which electromagnetic energy may leak or otherwise pass into or out of the device. Furthermore, such gaps represent discontinuities in the surface and ground conductivity of the housing or other shielding, and may even generate a secondary source of EMI radiation by functioning as a form of slot antenna. Any bulk or surface currents induced within the device housing develop voltage gradients across any interface gaps in the shielding thereby causing the gaps to function as antennae which radiate EMI noise.
One preferred shielding solution is to use a cover or housing shell made of metal to absorb and shield any EMI radiation energy generated by the electronic device. One drawback of such metal covers is additional weight and cost which is added to the electronic device. A further drawback is the inability and difficulty to form the metal sheet to a desired shape and contour of the electronic device housing thereby requiring the electronic device to be larger and less aesthetically pleasing than would otherwise be possible if not for the metal enclosure required for the EMI shielding. One proposed solution to reduce the weight and accommodate the shape and contour of the electronic device is to spray a metallized surface coating on the interior of a lightweight plastic or other suitable lightweight material forming the electronic device housing. Although this has the advantage of reducing the weight, the cost is increased due to the additional step of applying the metallized coating which complicates the manufacturing process and the cost of the metallic coating itself. Additionally, the metallized coating is easily scratched which reduces its shielding effectiveness.
A further problem is that the mating surface of the electronic device housing covers regardless of the material is not perfectly flat so that mating interfaces provide gaps from which EMI radiation energy can escape. Gaskets and other seals have been proposed for filling the gaps within mating surfaces of housings and other EMI shielding structures while maintaining electrical continuity across the structure. Such gaskets or seals are bonded or mechanically attached to or pressfitted into one of the mating surfaces and function to close any interface gap to establish a continuous conductive path thereacross by conforming under an applied pressure to irregularities between the surfaces. However, even pressure on the gasket interfaces may also generate gaps between the gasket surface and the ground layer and function as slot antenna to radiate EMI noise. In addition, the gaskets are also subject to shielding failures due to problems with compressibility, resiliency, and attachment. Further, there is an increasing demand in new products to make them smaller by reducing the number of screws, fastening devices and contact points which will require a more efficient and cost effective method for EMI shielding to accommodate these demands.
Accordingly it is an object of the present invention to provide an efficient and cost effective EMI shielding solution for portable electronic products particularly cellular and radio telephones.
In one aspect of the invention, an EMI shielding solution for a portable electronic device includes an electrically conductive fiber mesh net insert molded into the walls defining an interior cavity of the device to surround and shield first electronic circuitry contained within the device.
In a further aspect of the invention, at least a portion of the fiber mesh net is brought into direct continuous physical and electrical contact with a ground plane carried on a circuit board substrate within the electronic device.
In another aspect of the invention, an EMI shielding solution for a portable electronic device includes an electrically conductive fiber mesh net laminated to a surface of a polymer film sheet.
In a further aspect of the invention, the polymer film sheet has an electrically non-conductive surface opposite the fiber mesh net surface for carrying second electronic circuitry, and at least a portion of the fiber mesh net extends to the non-conductive side for mechanical and electrical coupling to the second electronic circuitry.
In a yet further aspect of the invention, the fiber mesh net is a mixture of conductive and non-conductive fibers with at least one of the conductive fibers carrying electrical signals from the first electronic circuitry on the printed circuit board to the second electronic circuitry.
In a yet further aspect of the invention, the fiber mesh consists of natural fibers or filaments such as cotton and other cellulose fibers, silk or other protein fibers and/or glass or other ceramic fibers.
In a yet further aspect of the invention, the fiber mesh consists of man-made synthetic, regenerated or metal fiber and filaments, such as polyesters, polyamides, polypropylenes, polyethylenes and cellulosics, and particularly suited are PES, PA6.6, PA6, PP and copper. Fibers, filaments, and yarns can be coated by a thin layer of a conductive metal layer thickness of 10-10000 nm, and preferably the conductive metal layer is a silver, nickel or aluminum layer. The fiber fineness can be varied from 0.05 den (0.055 dtex) microfiber to 100 den (110 dtex) monofilament whereas the yarn fineness can be extended according to the mesh type up to 300 den (33 tex).
In a yet further preferred aspect of the invention, the fiber mesh net consists of a bobbinet woven 3-directional net having 6 to 34 openings per inch and a specific weight of 10 to 50 grams per square meter.
In further aspects of the invention, the fiber mesh consists of warp knitted, woven, Raschel, braided nonwoven or spun multidirectional nets, respectively.